In modern optical networks, signals are often transmitted over hundreds, or even thousands of kilometers. Optical signals traveling over long-haul and ultra long-haul optical fibers may encounter many different obstacles, including attenuation, chromatic dispersion, and PMD. While attenuation problems have been successfully addressed by the use of amplifiers and chromatic dispersion by the use of dispersion compensating fibers, PMD has been much more difficult to handle.
PMD is a phenomenon that occurs when signals with different polarizations inside a fiber travel at slightly different speeds, for example, due to random imperfections and asymmetries of the optical fiber. This effect causes signal deformation. As a consequence, PMD can make it very difficult to reliably transmit data at high bit rates. Most networks were built with poor quality fibers in their underground installations at a time when low bit rates were used and PMD was not yet recognized as a potential issue. However, now that these structures must support bit rates of 40 Gb/s and higher, PMD presents a significant obstacle to network upgrading.
Generally, the PMD of an optical system cannot be accurately modeled by a single parameter (e.g., its length), but instead it must be characterized by a series of parameters that represent the entire “history” along the communication line. In practice, however, a few PMD measurement and correction systems have been developed. For example, U.S. Pat. No. 5,930,414 to Fishman, et al. and U.S. Pat. No. 6,865,305 to Rasmussen, et al. describe an electronic apparatus that measures eye-pattern parameters of a signal—e.g., signal-to-noise-ratio, error rate, crossing, etc., and thus indirectly determines the PMD of the optical line. Fishman, et al. further disclose correcting PMD by splitting the beam signal into two principal states of polarization (“PSPs”), subjecting one of the two PSPs to a relative delay using a delay line, and then recombining the two PSPs. Meanwhile, Rasmussen, et al. teach correcting PMD by using polarization maintaining fibers (“PMFs”).